A wide range of studies investigating the nature and determinants of radicalisation, and terrorist-related behaviour exist. These, in turn, have influenced theory, policy and practice in areas concerned with violent extremism prevention, disruption and management. As such interventions become more common, debates rage within mental health professions about the role mental health practitioners should play in countering violent extremism. This systematic review assesses the impact of mental health problems upon attitudes, intentions and behaviours in the context of radicalisation and terrorism. We identified 25 studies that measured rates of mental health problems across 28 samples. The prevalence rates are heterogenous and range from 0% to 57%. If we pool the results of those samples (n=19) purely focused upon confirmed diagnoses where sample sizes are known (n=1705 subjects), the results suggest a rate of 14.4% with a confirmed diagnosis. Where studies relied upon wholly, or in some form, upon privileged access to police or judicial data, diagnoses occurred 16.96% of the time (n=283 subjects). Where studies were purely focused upon open sources (n=1089 subjects), diagnoses were present 9.82% of the time. We then explore (a) the types and rates of mental health disorders identified (b) comparison/control group studies (c) studies that explore causal roles of mental health problems and (d) other complex needs.

Improvements have been made in identifying the prevalence of risk factors/indicators for violent extremism. A consistent problem is the lack of base rates. How to develop base rates is of equal concern. This study has two aims: (i) compare two methods for developing base rates; the Unmatched Count Technique (UCT) and direct questioning, (ii) generate base rates in a general population sample and compare these to a sample of lone-actor terrorists (n = 125). We surveyed 2108 subjects from the general population. Participants were recruited from an online access panel and randomly assigned to one of three conditions; direct survey, control, or UCT. Survey items were based on a loneactor terrorist codebook developed from the wider literature. Direct questioning was more suitable under our study conditions where UCT resulted in deflation effects. Comparing the base rates identified a number of significant differences: (i) lone-actor terrorists demonstrated propensity indicators related to a cognitive susceptibility, and a crime-and/or violence-supportive morality more often; the general sample demonstrated protective factors more often, (ii) lone-actor terrorists demonstrated situational indicators related to a crime-and/or violence-supportive morality more often, whereas the general sample experienced situational stressors more often, (iii) lone-actor terrorists demonstrated indicators related to exposure to extremism more often. Results suggest there are measurable differences in the prevalence of risk factors between lone-actor terrorists and the general population. However, no single factor "predicts" violent extremism. This bears implications for our understanding of the interrelation of risk and protective factors, and for the risk assessment of violent extremism.

A time sequence analysis is conducted on 125 lone actor terrorists, most of whom mounted attacks in Europe and North America, utilizing the TRAP-18 (Terrorist Radicalization Assessment Protocol), a structured professional judgment instrument with demonstrable interrater reliability and criterion, discriminant, and predictive validity. Both frequency filters (>3) and coefficient filters (>.50) were applied to the data. Results indicate that virtually all distal characteristics, such as criminal violence, mental disorder, and ideology, preceded the proximal warning behaviors, such as pathway, fixation, identification, leakage, last resort, and directly communicated threats. Indicators that were "gatekeepers" and "turning point events" were also identified (Taylor et al., 2008). The time sequence analysis further validates the model of the TRAP-18 as a risk instrument for the assessment and management of lone actor terrorist violence. Public Interest StatementThis study applies a terrorist risk assessment instrument, the TRAP-18, to a large sample of lone actor terrorists using time sequence analysis: a method by which indicators of risk are organized in a temporal relationship with one another. It allows counterterrorism investigators to better understand the unfolding of risk indicators over time on the way to a terrorist attack, and helps prioritize the most dangerous cases.

and Crime Science. Her research focusses on detecting dynamic patterns across a spectrum of grievance-fuelled violence to enhance the threat assessment of these types of offenders. She works as a research assistant on the ERC-funded Grievance project (see below).

Research Summary:The lone-actor terrorist population can be extremely heterogeneous and difficult to detect. Intelligence is vital to countering this threat. We devise a typology of person-exposure patterns (PEPs) that could serve as a framework for intelligence gathering and threat assessment. We use cluster analysis and a risk analysis framework (RAF) to identify relations among three components: propensity, situation, and network.The results of the analysis reveal four PEPs: solitary, susceptible, situational, and selection. The solitary PEP lacks common indicators of a propensity to pursue terrorist action. The susceptible PEP reveals cognitive susceptibility, manifesting as mental illness, to be a key factor in the emergence of a terrorist propensity. The situational PEP demonstrates how situational stressors may act as warnings of acceleration toward violent action. Lastly, the selection PEP demonstrates higher frequencies of leakage and antecedent violent behaviors.Policy Implications: Our findings have two key policy implications. First, given the multifinality of terrorism risk indicators, we suggest a move toward a structuredprofessional judgement approach to the risk analysis of lone-actor terrorists. Second, we present the PEP typology as a framework for intelligence gathering. Existing frameworks are predominantly focused on mobilization 452 CLEMMOW ET AL.indicators. We suggest expanding data collection to include propensity and situational indicators, as operationalized here, and using the PEP typology to inform decisions about the emergence of the motivation to commit an attack. To do so, it is necessary to pursue a multiagency approach to intelligence gathering.